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enter image description here

From this picture,when\$ V_{in} = V_{DD}\$,I know \$M_1\$ and \$M_3\$ will turn on,and \$M_2\$ will turn off because their \$ V_{gs} \$ are \$V_{DD}\$ and 0 ,but why will \$M_4\$ turn off,and \$V_B=V_{DD}\$, \$V_{out}=0\$ ?

If i know \$V_{out}=0\$ and \$V_B=V_{DD}\$ first,then of course i know \$M_4\$ will turn off ,but if now i don't know the value of \$V_B\$ and \$V_{out}\$,and neither do \$M_4\$ ,how do i know \$M_4\$ turn off,and \$V_B=V_{DD}\$, \$V_{out}=0\$ ?

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    \$\begingroup\$ How about recognizing good answers on previously raised questions or do you think you deserve free advice? \$\endgroup\$
    – Andy aka
    Feb 2, 2018 at 14:56
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    \$\begingroup\$ i had did it,but some question is not the right answer \$\endgroup\$
    – Shine Sun
    Feb 2, 2018 at 14:58
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    \$\begingroup\$ but some question is not the right answer Hmm, does that mean you already know what the right answer is? \$\endgroup\$ Feb 2, 2018 at 15:05
  • \$\begingroup\$ why M4 and M3 gates are floating ?? \$\endgroup\$ Feb 2, 2018 at 15:19
  • \$\begingroup\$ @TonyStewart.EEsince'75 That's what I thought as well but it is just drawn in a bad way. That node is actually \$V_{IN}\$. A simple vertical line to the \$V_{IN}\$ text would have fixed that, like is done at the left side of the circuit. \$\endgroup\$ Feb 2, 2018 at 15:22

2 Answers 2

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As the gate of M4 (which is a PMOS) is \$V_{DD}\$, the only way to make M4 turn on is the take the bulk or drain or source connection of M4 at least \$V_t\$ above \$V_{DD}\$. So we would need \$V_{DD} + V_t\$ to be present somewhere in the circuit.

This can only happen if capacitor \$C_B\$ is first charged (with \$V_A\$ = GND and \$V_B\$ = \$V_{DD}\$) and then discharged by making \$V_A = V_{DD}\$ then there would for a short time be \$2*V_{DD}\$ present at \$V_B\$. But as \$C_B\$ discharges through M5, which is on, see Andy's answer, so the \$V_{GS}\$ of M4 must be zero, hence it is off.

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If M3 turns on then M5 has to activate and this has to turn M4 off. If M3 is on and M4 off then Vout is zero.

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  • \$\begingroup\$ why does M5 need to activate when M3 turn on? \$\endgroup\$
    – Shine Sun
    Feb 2, 2018 at 15:10
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    \$\begingroup\$ M5 activates because M3 is shorting M5's gate to zero volts and M5's source is high. \$\endgroup\$
    – Andy aka
    Feb 2, 2018 at 15:20

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